Dr. Krishna Sinha, emeritus professor at Virginia Tech and adjunct professor at UCSB, will give a talk at Speakers Club entitled The nature of magmatism associated with breakup of super-continents.

ABSTRACT:

The assembly of supercontinents and their subsequent dispersals is one of the most challenging research frontiers in earth science. The duration of supercontinent cycles, including the creation of supercontinents as well as the causes associated with their fragmentation have been linked to distinct mantle processes such as slab avalanche, mantle upwelling, super-plumes and plate boundary driving forces. The central Appalachian region provides a geologic record of the assembly and dispersal of two supercontinents, Rodinia and Pangaea. Associated with the breakup of  each supercontinent  Large Igneous Provinces (LIPS) are represented by the Catoctin Volcanic Province (CVP; associated with breakup of Rodinia) and a younger Mesozoic Central Atlantic Magmatic Province (CAMP; associated with breakup of Pangaea). Isotopic and trace element data for CVP from measured stratigraphic sections, as well as dikes and amphibolites hosted in sedimentary basins developed during extension, are used to provide  temporal and spatial monitoring of compositional diversity associated with  rift to drift transition. The data highlight changes in source regions producing an early low titanium suite, followed by emplacement of high titanium magmas, and finally basalts derived from a depleted mantle source. This orderly compositional change is recognizable even in a complex structural setting.

Comparisons of  geochemical and isotopic similarities and differences between the two LIPS are useful in monitoring mantle processes in two temporally distinct but spatially co-located large igneous provinces. For example, a comparison of incompatible element ratios between CVP basalts (e.g., Zr/Y = 4.91, La/Yb = 5.73, Th/Ta = 1.37 and Hf/Th = 2.82) and CAMP (e.g., Zr/Y = 2.30, La/Yb = 2.16, Th/Ta = 6.97 and Hf/Th = 0.96) suggests that similar sources were not sampled during the melting episodes for the two provinces. These data  suggest that voluminous high titanium CVP  magmas were derived from melting of an OIB (plume?)  that had undergone geochemical interaction with the crust. In contrast, CAMP magmas have been modeled as melts derived from interaction of  a depleted mantle with crustal rocks with no recognizable  plume component.